The present invention relates to the use of subatmospheric pressure to expedite refining of molten glass or the like. More particularly, the invention relates to a selected rate and extent of foaming in such a refining technique that yields improved refining performance.
In the melting of glass, substantial quantities of gas are produced as a result of decomposition of batch materials. Other gases are physically entrained by the batch materials or are introduced into the melting glass from combustion heat sources. Most of the gas escapes during the initial phase of melting, but some becomes entrapped in the melt. Some of the trapped gas dissolves in the glass, but other portions form discrete gaseous inclusions known as bubbles or "seeds" which would be objectionable if permitted to remain in unduly high concentrations in the product glass. The gas inclusions will rise to the surface and escape from the melt if given sufficient time in the stage of a melting operation known as "refining" or "fining." High temperatures are conventionally provided in the refining zone to expedite the rise and escape of the gaseous inclusions by reducing the viscosity of the melt and by enlarging the bubble diameters. The energy required for the high temperatures employed in the refining stage and the large melting vessel required to provide sufficient residence time for the gaseous inclusions to escape from the melt are major expenses of a glassmaking operation. Accordingly, it would be desirable to assist the refining process to reduce these costs.
It has been known that reduced pressure could assist the refining process by reducing the partial pressure of the included gaseous species and by increasing the volume of bubbles within the melt so as to speed their rise to the surface. The impracticality of providing a gas-tight vessel on the scale of a conventional refining chamber so as to draw a vacuum therein has limited the use of vacuum refining to relatively small scale batch operations such as disclosed in U.S. Pat. Nos. 1,564,235; 2,781,411; 2,877,280; 3,338,694; and 3,442,622.
Continuous vacuum refining processes have been proposed but have not found acceptance for large scale, continuous manufacture of glass due to various drawbacks. In the continuous vacuum refining arrangements shown in U.S. Pat. Nos. 805,139; 1,598,308; and 3,519,412 a disadvantage is the requirement for relatively narrow vertical passageways leading into and out of the vacuum zone necessitated by the pressure difference. Also, the molten glass is not fully exposed to the vacuum since the incoming glass enters from below a pool of glass.
A different arrangement is shown in U.S. Pat. No. 3,429,684, wherein batch materials are fed through a vacuum lock and melted at the top of a vertically elongated vacuum chamber. Melting raw materials within the vacuum chamber is a disadvantage of that arrangement for three reasons. First, large volumes of foam would be created by carrying out the initial decomposition of the raw materials under vacuum, which would require a vessel large enough to contain the foam. Second, there is a danger that raw materials may follow a short circulation path to the output stream, thus avoiding adequate melting and refining. Third, carrying out the initial stages of melting and heating the melt to a refining temperature within the vacuum vessel require large amounts of heat to be supplied to the melt within the vessel. Such a major heat input to the vessel inherently induces convection currents within the melt that increase erosion of the walls, which leads to contamination of the refined product stream.
U.S. Pat. No. 4,195,982 discloses initially melting glass under elevated pressure and then refining the glass in a separate chamber at a lower pressure. Both chambers are heated.
A preferred technique for vacuum refining glass is disclosed in U.S. Pat. No. 4,738,938 (Kunkle et al.) wherein the creation of foam is deliberately enhanced by introducing the molten glass into the vacuum chamber above the level of the molten glass held therein. Excessive foam was indicated in that patent as being a problem to be avoided. A large space above the liquid container must be provided to accommodate the foam if a large throughput is desired. Since this headspace must also be maintained gas-tight, its construction can be a significant economic drawback, particularly on a large scale process. As a result, the volume of foam acts as a limiting factor to the thoughput rate and/or the degree of vacuum that can be utilized.
Bronze or gray colored glasses sometimes include selenium as an essential colorant. Examples of heat-absorbing architectural glasses of this type are disclosed in U.S. Pat. Nos. 2,938,808 (Duncan et al.) and 3,296,004 (Duncan). It has been found that when these selenium-containing glasses are subjected to vacuum refining in accordance with the preferred techniques so much of the selenium is removed from the molten glass that insufficient selenium is retained in the glass product to provide the desired coloration. This problem exists even when large amounts of excess selenium are provided in the batch mixture. It would be desirable to use vacuum refining to produce selenium-containing glass.